Electronically-commutated synchronous machines with a permanently-excited rotor and a stator provided with phase windings, also known as brushless dc motors, enjoy great popularity. They are characterized by low wear, and by their limited electromagnetic or acoustic interference, and show a high degree of efficiency. The influx of current to typically three stator windings generates a magnetic field, in which the permanently-excited rotor in arranged. By the appropriate switchover (commutation) of the currents flowing in the stator windings (or of voltages applied), a rotary motion is generated. Electronically-commutated motors are also used as drive mechanisms for electromechanically-actuated wheel brakes of a motor vehicle, wherein an appropriate linkage mechanism converts the rotary motion of the motor into a translational motion of the brake linings. For the purposes of such application, motors are designed with a high torque rating at rest or at low speeds of rotation.
In order to achieve electronic commutation of the motor, the position of the rotor must be known; for a motor at rest, or a motor rotating at low speed, the rotor position may be appropriately determined by means of sensors. In a widely-applied arrangement, three Hall-effect sensors fixed to the stator are used for the scanning of a rotary encoder associated with the rotor, or for the direct scanning of the stray field of the rotor magnets.
WO 2005/046041 A2 describes a method for the operation of a synchronous machine with a permanently-excited rotor, a stator provided with phase windings and a non-absolute position sensor, whereby the rotor start position is determined. In off-load synchronous machines, for the elimination of angle errors in the determination of the rotor start position, a minimum of one current and/or voltage vector is delivered in the direction of the determined rotor position for a specific time interval, thereby forcing the arrangement of the rotor to the position concerned. Although this method permits the elimination of minor angle errors, it does not permit the reliable detection of a defective sensor.
If the rotor position detected by the sensors is not consistent with the actual rotor position, malfunctions of various types may ensue. In many cases, this may result in damage to the motor, the driven device or the motor control system.
Accordingly, the object of the present invention is to verify the operation of the rotor position sensors and, in case of the correct operation of the sensors, to detect any deviation between the actual rotor position and that indicated by the sensors, and to generate an angular correction for the control of the motor.
This object is fulfilled by the method according to this invention.
A method is therefore provided in accordance with this invention wherein the relative position of the rotor of an electronically-commutated synchronous machine to one or more, specifically three rotor position sensors which are fixed to the stator is determined. After a first rotor position determination process, the off-load rotor is rotated through a predefined angle by the application of an appropriate voltage vector. Thereafter a further rotor position determination process is carried out, and the result of the second rotor position determination process is compared with an anticipated rotor position. After the first position determination process, during rotation through a fixed electrical angle, e.g. 60°, an anticipated rotor position may be determined. In case of the coincidence of the anticipated and measured second rotor position, the positional error lies below the angular resolution defined by the number and position of the rotor position sensors in the rotor position sensor system.
Preferably a first commutation scheme is specified, whereby an association between the voltage vector applied and the anticipated rotor position is established. If a commutation scheme is specified, the anticipated rotor position may be determined by means of a simple read-out from the commutation table.
It is advantageous if the association between the current rotor position and the voltage vector to be applied for the achievement of a desired rotation is determined by means of a specified first commutation scheme. In a specified commutation scheme, rotation through a specific angle may be achieved to an accuracy which is determined by the arrangement and number of sensors, by the application of a voltage vector which differs from the commutation table by an angle-related number of columns.
Preferably, by reference to the comparison between the result of the second rotor position determination process and the anticipated rotor position, all subsequently determined rotor positions and derived variables, including e.g. the instantaneous speed of rotation, are corrected. Should any displacement in the rotor position be detected by the rotor position sensors during the subsequent commutation of the motor, the highest possible mechanical torque will continue to be delivered.
During the determination of the rotor position, the currents flowing in the phase windings are subject to appropriate limitation on the basis of temperature information. Specifically, in case of a higher temperature, a higher limiting current is selected and, in case of a lower temperature, a lower limiting current is selected. At a higher temperature, the reduced magnetization of the permanently-excited rotor is offset by a correspondingly increased current flow in the stator windings. The coil currents may be selected to a sufficiently high rating to permit the execution of the method according to the invention. As a result of the limited torque delivered, the risk of any damage to the motor or the driven device is minimal.
During the determination of the rotor position, it is advantageous if the currents flowing in the phase windings are limited by the alternating cut-in and cut-out of the voltages applied to the phase windings. This permits current limitation, even where the control electronics of the motor include no provision for pulse-width modulation.
In a specifically preferred form of embodiment of the invention, the electronically-commutated synchronous machine is provided with a permanently-excited rotor and a stator which is provided with three phase windings, together with three rotor position sensors in an appropriate arrangement for a first commutation scheme. The rotor position is determined by the read-out of data from the position sensors, the appropriate voltage vector is defined by a second commutation scheme, and the second rotor position thus determined is compared with the first commutation scheme. If the rotor position sensors are operating correctly, the position of the rotor may be determined to an accuracy which is equivalent to the interval between two columns in the commutation table.
In a particularly preferred form of embodiment of the invention, rotor positions are expressed as angles, wherein 360° represents one complete cycle of a commutation scheme and, after the comparison of the second determined rotor position with the first commutation scheme, a voltage vector with an angular displacement of 90° in the first commutation scheme is applied, whereafter a voltage vector in the second commutation scheme at an angle displaced by a further 90° in the same direction is applied, and a third rotor position is determined thereafter, whereby an error is detected if the data from the position sensors for the third rotor position are not inverted in relation to the data for the second rotor position. If these data are inverted, the correct operation of all the rotor position sensors is confirmed.
According to an alternative and particularly preferred form of embodiment of the invention, at least one further voltage vector, adjoining in the opposite direction of rotation, of the second commutation scheme is applied thereafter, and a third rotor position is determined thereafter, whereby an error is detected if the data from the position sensors for the third rotor position do not differ from the data for the second rotor position.
In a particularly preferred arrangement, the electronically-commutated synchronous machine is provided with a permanently-excited rotor and a stator which is provided with three phase windings, together with three rotor position sensors in an appropriate arrangement for a commutation scheme. Prior to the initial determination of a rotor position, a base voltage vector for the commutation scheme is applied, the rotor position is determined by the read-out of data from the position sensors, the appropriate voltage vector is varied in relation to the base voltage vector by specific increments, and the second determination of the rotor position, together with the adjustment of the appropriate voltage vector, are continued progressively until an adjoining rotor position is indicated. This method permits the more accurate determination of the rotor position or of the switchover point of the rotor position sensors.
In a particularly preferred arrangement, the rotor position sensors operate on the basis of the Hall effect; the incremental variation of the voltage vector and the read-out of data from the position sensors are repeated, whereby the desired rotor position is achieved, in both running directions of the motor, by the application of the base voltage vector, the existing voltage vectors at the time of a change to the data from the position sensors are compared with the base voltage vector, and a resulting measure is calculated for the hysteresis of the position sensor operating by the Hall effect, the data read-outs for which have changed.
Specifically, the hysteresis of all position sensors operating by the Hall effect is determined by the application of a base voltage vector which is adjacent to the relevant switchover point, and the repeated incremental variation of the voltage vector with a simultaneous read-out from the position sensor concerned.
In a particularly preferred arrangement, the switchover positions of the rotor position sensor(s) concerned, which are adjusted in relation to the base voltage vector, are considered by the application of appropriate adjustments to the subsequent control of the electronically-commutated synchronous machine whereby, specifically, a positional difference and/or hysteresis is logged and taken into consideration for each rotor position sensor in question.
In a particularly preferred arrangement, the appropriate voltage vectors are set by the use of known vector modulation methods, whereby dead-time effects in particular are compensated by the appropriate adjustment of the relevant pulse duty factor. Dead-time effects are generated by the requisite time delay associated with the connection of a rectifier bridge, in order to prevent any short-circuit. By the adaptation of the pulse duty factor, i.e. the ratio of the pulse width to the pulse interval, the generation of a voltage vector with a defective angle can be avoided.
According to an alternative, particularly preferred form of embodiment of the invention, the appropriate voltage vectors are varied by the adjustment of one of the voltages applied, by means of the pulse duty factor.
In a further particularly preferred form of embodiment of the invention, the appropriate voltage vectors are generated by means of rapid switchover between adjacent base voltage vectors.
It is particularly expedient if a limitation of current is effected by the repeated and short-term input of zero vectors. Under zero vector conditions, all windings are at the same potential, such that no voltage difference is generated.
The invention also relates to a circuit arrangement for the control of an electronically-commutated synchronous machine, which executes a method whereby the position of the rotor of an electronically-commutated synchronous machine relative to one or more, specifically three rotor position sensors which are fixed to the stator is determined and which, specifically, is a constituent element of a circuit arrangement for the control or regulation of the brakes of a motor vehicle.
Appropriately, on-load operation of the motor proceeds only after the error-free execution of a method whereby the position of the rotor of an electronically-commutated synchronous machine relative to one or more rotor position sensors is determined whereby, specifically, the control system considers any difference detected between the actual and notional position of the rotor.
The invention also relates to the use of a circuit arrangement of this type in motor vehicles.
The currents flowing in the stator windings are preferably limited, specifically for the prevention of damage to the motor or the driven device.
In a particularly preferred arrangement, current limitation is achieved by the limitation of the voltage applied, specifically by means of pulse width modulation in the control circuit.
In a further and particularly preferred form of embodiment of the invention, the flow of current is subject to near-total interruption when a temperature threshold is exceeded.